The current 8 series chipsets, C1, has a bug in its integrated USB 3.0 controller. Connected USB 3.0 devices based on one or more controllers - it's unclear exactly which one - will briefly disconnect and reconnect when a PC wakes up from standby (S3). This can pose a problem if you had a file open that's located on a USB 3.0 storage device, as the software will tell you that the file cannot be found after your computer comes out of standby mode.

The problem is not with the Haswell processors, but with the 8 series chipsets

Intel sees the issue as annoying but not as a deal-breaker, and we tend to agree. We're able to reproduce the problem, but can't really think of many scenarios in which it will be huge problem. All you have to do is to open the file again, and no data can be lost.

If you want to be sure that you buy a motherboard without that issue, you will have to wait. Intel has indicated that, starting end July, a new version of the chipset, C2, will be supplied to the motherboard manufacturers. Then the motherboards have to be produced and shipped, and retailers will of course first sell their existing stock.

You can't tell by the exterior whether a board is C1 or C2. The manufacturers are also of the opinion that it's insignificant and we haven't heard any plans that it will be mentioned specifically on the box which version it is. Once you've bought a board, you can find out which one you have with CPU-Z. Under the tab Mainboard you can see the revision of the chipset. Revision 4 is C1, and the improved C2 stepping will be called revision 5.